Mesa Boogie Mark III - reverb out

[vrdyer]

The reverb on my Mesa Mark III went out. Changed tubes, check connections, tried changing rca cords on the reverb, pulled the tank and checked for wiring problems, cleaned the eq/reverb jack that I don't use.... Suggestions, recommendations?

 

[jpdennis]

Did you try a known good tank just to see if it is your tank or at least put your exisitng tank into another amp which uses that type 9? Also, did you clean the reverb pot and check it with a meter while rotating it from zero to 10?

Dennis

 

[dgr888]

If you have a meter check the input and output on the tank which are usually the green and black wires attached to the rca jacks. Check across the green and black wires of each on a low scale of your meter and you should get a reading. If you don't get a reading the small reverb transformer is open or the green or black wire has a bad connection at the jack. I have also seen these small wires break close to the small transformer which can be a real pain to fix.....Good Luck

 

[gts]

You probably have a grounding problem. Happens all the time.
All works fine, then poof it stops for no apparent reason.
Do this:
Then report back.

 

[dgr888]

This fix that Mesa put out actually introduces MORE hum into their already noisy reverb circuit and DOESN'T fix anything.... don't do it unless you want more noise when you find the real problem and get it working again.

 

[jpdennis]

This fix that Mesa put out actually introduces MORE hum into their already noisy reverb circuit and DOESN'T fix anything.... don't do it unless you want more noise when you find the real problem and get it working again.

And I thought this was for the Mark II series. Since the Mark III amp output to the tank (the tank's input) is supposed to not be grounded per the tank part# & specs.

Dennis

 

[dgr888]

>snip<

 

[MrMarkIII]

Yep, that pesky misinformation can be troublesome.
The factory spec reverb tank for a Mark III (Three) is:
9AB2A1B.
The NON-mis-information below is from Accutronics.
(They make the tanks):

PART NUMBERING SPECIFICATIONS FOR TYPE 9

EXAMPLE: 9AB2A1B (The correct original tank spec for a Mesa Mark III)

DIGIT #1 - REVERB TYPE
9 for Type 9

DIGIT #2 - INPUT IMPEDANCE
A = 10 Ohm
B = 190 Ohm
C = 240 Ohm
D = 310 Ohm
E = 800 Ohm
F = 1925 Ohm

DIGIT #3 - OUTPUT IMPEDANCE
A = 600 Ohm
B = 2575 Ohm
C = 12000 Ohm

DIGIT #4 - DECAY TIME
1 = Short (1.2 to 2.0 sec)
2 = Medium (1.75 to 3.0 sec)
3 = Long (2.75 to 4.0 sec)

DIGIT #5 - CONNECTORS
A = Input Grounded / Output Grounded
B = Input Grounded / Output Insulated
C = Input Insulated / Output Grounded
D = Input Insulated / Output Insulated
E = No Outer Channel

DIGIT #6 - LOCKING DEVICES
1 = No Lock

DIGIT #7 - MOUNTING PLANE
A = Horizontal Open Side Up
B = Horizontal Open Side Down
C = Vertical Connectors Up
D = Vertical Connectors Down
E = On End Input Up
F = On End Output Up

Please note section highlighted in RED.
QED.
You're welcome.

 

[gts]

This fix that Mesa put out actually introduces MORE hum into their already noisy reverb circuit and DOESN'T fix anything.... don't do it unless you want more noise when you find the real problem and get it working again.

Just trying to help the guy get his reverb up an running again
as he was concerned about having no reverb (versus hum/ noise)
It may be a 'lack of a ground' problem so therefore the fix above works in some of these situations.

Fwiw pretty sure the Accutronic 9AB2A1B tank has been the stock tank for the Mark series for years.

 

[jpdennis]

Yep, that pesky misinformation can be troublesome.
9AB2A1B (The correct original tank spec for a Mesa Mark III)

DIGIT #5 - CONNECTORS
A = Input Grounded / Output Grounded

Please note section highlighted in RED.
QED.
You're welcome.

+1 MrMarkIII

Thanks, I checked the schematic and misread the tank. I now need to pull my tank to see if I didn't foul something up or just was confused.

Dennis

 

[dgr888]

>Snip<

 

[JOEY B.]

Yep, that pesky misinformation can be troublesome.
9AB2A1B (The correct original tank spec for a Mesa Mark III)

DIGIT #5 - CONNECTORS
A = Input Grounded / Output Grounded

Please note section highlighted in RED.
QED.
You're welcome.

+1 MrMarkIII

Thanks, I checked the schematic and misread the tank. I now need to pull my tank to see if I didn't foul something up or just was confused.

Dennis

To quote Whitney Houston, "crack is whack".

The reverb tanks' metal chassis is connected to BOTH the input and output RCA jacks outer terminals on the 9AB2A1B internally. The white wire of the factory reverb cables is a single conductor that only transports signal and NO ground or shield. The one-wire mod only adds a supplemental ground to the grey wire when the ground connection is lost at its chassis attachment. NO ADDITIONAL NOISE CAN BE PRESENT, AS THERE ARE NO GROUND LOOPS ADDED. I verified all of this with a multimeter on my Mark II head that has the one-wire mod. NO CONTINUITY at the reverb cable (ground) ends that plug into the tank when the cables are unplugged from the reverb tank.

If my post is confusing, revert back to the "single conductor white wire comment". Rinse and repeat. :shock:

 

[dgr888]

I know from repairing these since the 80's when this fix came out it caused and still causes a ground loop/condition which IS fedback thru the ground not the single conductor. Noise travels between grounds when they are at different potentials. The problem is the grounding design scheme on the board itself so when the input and output ground are tied together with the "one wire" method it creates an albeit slight, but nonetheless increased noise floor. Check it with a scope and it's there. Some members use these in the studio or at low levels and want absolutely no noise present. You can use the "one wire" fix a broken ground but at a cost of an increase in noise/hum etc...

 

[vrdyer]

Thanks to all of you! I actually contacted Mike B at Mesa today. Considering sending it his way. Never has been serviced since it was new I bet. I am the second owner. Found it on the Recycler in CA for $350. What a deal! Yeah, I know. Well, it arrived in a fruit box, lightly taped, no padding. Yes I called the seller, but he would never answer or respond to snail mail. Tubes were mostly gone. Some of the knobs on the sliders were missing. Logo was bent with picks stuffed underneath it. It was described as in very good condition on the phone before the sale and in his internet add. No issues he said.

I cleaned, re-soldered, re-glued, re-knobed, re-tubed, and removed the gunk. It sounded great. Amazement. It has played well for more than 12 years. Mike said to box it like it might withstand a 4 foot drop. He wasn't joking.

Did you know..... Mesa did cut holes in the front of the heads to install the fans (hole is covered by screen). The fan was tacted down. I thought the previous owner did that. Not too bullet-proof.

Arrived home late and I have not had time yet, but I will indeed try to match up the grounds. That fix makes since to me. We will see if that works.

Just a comment...this amp is a survivor. I've played it outside with two 412s with the sound loud enough to hurt if you walked in front of it. I was only drowned out when one of the bikers started up and drove off. It's withstood brownouts (yeah just like the add) when the Marshalls next to me shorted, short drops, lots of knocking around. The loop has never worked right for me. Always volume loss and treble/presence loss when I used it. Switching pedals are sturdy and the replacement cables are any guitar cable. Scratchy pots seemed easy to correct.

Ramble ramble.

 

[gts]

<snip>

 

[gts]

Hey vrdyer you got a great deal. Congrats!
After you send it to Mesa and have Mike B go over it, when all is said and done (the cost to ship and for the repair/ general maintenance), you'll still have great deal.
Enjoy!

 

[dgr888]

Great deal hope you can get it running good...

 

[JOEY B.]

I know from repairing these since the 80's when this fix came out it caused and still causes a ground loop/condition which IS fedback thru the ground not the single conductor. Noise travels between grounds when they are at different potentials. The problem is the grounding design scheme on the board itself so when the input and output ground are tied together with the "one wire" method it creates an albeit slight, but nonetheless increased noise floor. Check it with a scope and it's there. Some members use these in the studio or at low levels and want absolutely no noise present. You can use the "one wire" fix a broken ground but at a cost of an increase in noise/hum etc...

Could it be, what you are seeing on the scope, is noise inducted into the jumper wire from the power transformer? I still say that the one-wire fix just gives the reverb tank back the ground that it lost due to an oxidized washer, and that the fact that it is only one circuit negates a ground loop. No noise present? It is a tube amp, right? :roll: Also, playing a Boogie at low volumes is the equivalent of driving a Ferrari in a school zone, for me anyway. :wink:

 

[dgr888]

The drive side of the reverb circuit and the recovery side are two separate circuits not one.. Tube amps can be designed to be quiet, it's just we are accustomed to and made to think it is normal for them to have noise. And yes to some it doesn't matter but to some it does. For the price paid there shouldn't be any bugs to be fixed/and or noise.

 

[JOEY B.]

The drive side of the reverb circuit and the recovery side are two separate circuits not one.. Tube amps can be designed to be quiet, it's just we are accustomed to and made to think it is normal for them to have noise. And yes to some it doesn't matter but to some it does. For the price paid there shouldn't be any bugs to be fixed/and or noise.

OK, you win. I've worked 19 (10 hour) days in a row, I'm tired. I have 3 Mark II amps, one without the mod and two with. I have NEVER noticed more noise in the amps with the mod. I will also put up with a little bit of noise to have guitar tone to my liking (tube amp). 8)